Extremely is an adverb used to express a very high degree or intensity of something. It commonly modifies adjectives and other adverbs, indicating emphasis or extremity in meaning.
Grammar.com’s section on the Parts of Speech highlights discussions about the gradual decline of adverbs ending in -ly in modern English. Despite this trend, extremely remains standard and widely accepted in both formal and informal contexts.
The weather was extremely cold last night.
In this case, the adverb intensifies the adjective cold, showing a strong degree.
Author’s summary: The word extremely functions as a common adverb denoting high intensity and continues to hold its place in contemporary English despite shifting usage patterns.